Years of research and development efforts have led to significant achievements in the nanoscale electronics field. Nanoscale electronics generally refer to electronic circuits, fabricated by any of various methods, which include signal lines with widths less than 100 nm and active and passive electronic components, each fabricated from one to tens or hundreds of molecules. In one promising nanoscale electronic circuit architecture, nanowire crossbars are fabricated from a first set of closely spaced, parallel nanowires overlaid by a second set of closely spaced, parallel nanowires, with electronic components, such as diodes, resistors, and passive connections fabricated within the overlap regions, or nanowire junctions, where nanowires from the first set of nanowires cross nanowires from the second set of nanowires.
FIGS. 1A–C illustrate a simple, generic nanowire crossbar. FIGS. 1A–C all employ the same illustration conventions, described only with reference to FIG. 1A, for the sake of brevity. In FIG. 1A, a first set of parallel nanowires is represented by vertical lines 102–105 and a second set of parallel nanowires is represented by horizontal lines 106–108 and 110–111. The two sets of parallel nanowires, together with nanoscale electronic components selectively fabricated at certain nanowire junctions between nanowires of the first and second sets, compose an example nanowire crossbar 100. In FIG. 1A, nanoscale electronic components are represented by disks 112–117. The nanoscale electronic components include nanoscale diodes, nanoscale resistors, nanoscale connection points, and nanoscale transistors.
FIG. 1B shows operation of an electronic circuit implemented by a nanowire crossbar. A first group of the second set of nanowires, such as nanowires 106–108 in the example nanowire crossbar of FIGS. 1A–C, may serve as input signal lines, and a second group of the second set of nanowires, such as nanowire 110 in the example nanowire crossbar of FIGS. 1A–C, may serve as output signal lines. The example nanowire crossbar 100 of FIGS. 1A–C implements a nanoscale logic circuit into which a 3-bit signal is input and from which a 1-bit signal is output
Nanowire crossbars generally include from tens to hundreds of parallel nanowires in each of the first and second parallel nanowire sets, which may be partitioned more or less arbitrarily between input, output, and internal signal lines. This partitioning occurs by interconnection of the nanowires to additional signal lines and circuitry. The logic function of the nanowire crossbar is determined by selective fabrication of nanoscale electronic components 112–117 at particular nanowire junctions within the nanowire crossbar. In certain types of nanowire crossbars, the nanowire junctions may be reprogrammable. The electronic circuit implemented by reprogrammable nanowire crossbars can therefore be repeatedly redefined. As shown in FIG. 1B, when the input “101” is encoded as high and low voltages and input to input signal lines 106–108 of the example nanowire crossbar illustrated in FIGS. 1A–C, the signal “1” is output on output signal line 110.
FIG. 1C shows a different logical-input/logical-output pair from that shown in FIG. 1B. In FIG. 1C, the example nanowire crossbar transforms the input signal “100” to output signal “0.” A full logic description for the example nanowire crossbar would consist of a table of possible logical-input/logical-output pairs, or, alternatively, one or more Boolean expressions from which the possible logical-input/logical-output pairs can be derived. In general, a nanowire crossbar can be used to implement arbitrary logical circuits that transform an input logic signal of an arbitrary width, in signal lines or bits, to an output logic signal of an arbitrary width.
Nanowire crossbars employing diode-like nanoscale components at nanowire junctions have been proposed as important logic components of a new generation of nanoscale electronics. Diode-resistor logic inherently degrades signals because of voltage drops across diodes and pullup and pulldown resistors. Cascading diode logic produces cumulative signal degradation that, when diode logic is cascaded to any significant depth, produces sufficient signal degradation to make such cascaded logic circuits unusable. Since signal degradation is cumulative through each and every diode-resistor logic stage, amplification of degraded signals between diode-resistor logic stages is needed to restore signal integrity. Diode-resistor logic also lacks the ability to store logic states, so that implementation of sequential diode-resistor logic is difficult.